The moon is not about to change its size. Almost since its formation more than four billion years ago, it has maintained a mean diameter of 2,159.2 miles. That’s less than the distance from Los Angeles to New York. As NASA puts it, if the Earth were the size of a nickel, the moon would be about the size of a coffee bean. But that doesn’t mean the moon can’t look bigger or smaller—and this Saturday, April 12, it will be comparatively tiny, amounting to what sky watchers call a micromoon. Here’s what you need to know.
On any given night, the moon can appear to change its size—sometimes dramatically. Thanks to the so-called moon illusion, it may appear especially large—sometimes huge—when it is low in the sky, hugging the horizon, and then appearing to shrink steadily as it climbs in the sky. But your eyes and, significantly, your brain, are lying. To prove it, simply stretch your arm out, close one eye, and hold your index finger up. Your fingernail and the moon will appear about the same size, regardless of how big or small the moon looks. What’s going on is a trick of the mind; at the horizon, you make an unconscious comparison between the size of the moon and the size of objects like houses and trees, with the moon appearing to dwarf them. High overhead the moon is the one that gets dwarfed by the emptiness of the sky. Your mind thus concludes that it has shrunk.
The micromoon is something else. On average, the moon is 238,855 miles from Earth, but that figure changes. The moon’s orbit around the Earth is elliptical, with a low point, or perigee, of about 224,000 miles, and a high point, or apogee, of 251,650 miles. When we see the moon at perigee, closer to Earth, it naturally looks bigger—creating the phenomenon known as a supermoon, when the lunar disk appears 7% larger and 15% brighter than it typically does. At apogee—which is where the moon will be this weekend—it will appear 14% smaller and 30% dimmer, creating the illusion of the micromoon.
This micromoon will also be known as a pink moon, though that has nothing to do with its color. (That’s unlike the blood moon, which occurs during a lunar eclipse, when the Earth’s shadow falls on the moon, and the blue wavelengths of sunlight streaming past our planet are scattered by the atmosphere while the red wavelengths pass through, tinting the moon.) The pink moon is just a nickname for the full moon that occurs in April, as a nod to the flowers that bloom in the season.
The pink micromoon will begin rising at dusk this Saturday, reaching its greatest illumination at 8:22 PM EDT. Space enthusiasts will be looking upward then, just as they do for a supermoon. But not everyone gets so excited. As no-less an authority than astronomer Neil deGrasse Tyson tweeted back in 2017: “If last month’s Full Moon were a 16.0 inch pizza, then this month’s ‘Super’ Moon would be 16.1 inches. I’m just saying.” By that measure, the micromoon will be a 13.76-inch pie. Enjoy it all the same.
